1. Field of the Invention
The invention relates to binary circuitry including switching elements utilizing superconductive tunneling effects, for storing and/or connecting binary coded data logically.
Binary circuits can be used in all kinds of apparatus and devices for processing binary coded date. These may be computer systems, telephone central offices, or any other system for the transmission of binary data. If the switching behavior of such circuits is bistable, one can design therefrom memories, shift registers, counters or like apparatus. If the switching behavior of such circuits is monostable, they reset automatically to their initial state after cessation of input signals. Particularly such binary circuits are useful in designing logical connective circuits of all kinds.
2. Description of the Prior Art
The term superconductivity means the complete disappearance of electrical sensitivity of a number of metals and alloys at very low temperatures near absolute zero. Superconductivity occurs suddenly below a certain temperature. This so called critical temperature differs from material to material. Binary switching elements known as cryotrons utilize the transition of the material from the superconducting state to the normal state and vice-versa. The alternate generation and destruction of superconductivity by controlling currents or magnetic fields effects a phase transition in at least one of the materials of the cryotron. This phase transition starts at nucleation centers and spreads over the cross section until the whole material exists in the new phase. Of course, such a phase transition consumes a certain period of time and such transients proceed relatively slowly when compared with known devices. A serious drawback of the cryotron is also the fact that the material exhibits an ohmic resitivity in the normal conducting state, and, therefore, the power dissipation can no longer be neglected.
If two superconductors are separated by a thin, nonsuperconducting layer, electrons may cross that potential threshold by the action of driving fields, although such a threshold should not be possible to overcome, strictly speaking. Electrons cross it by tunneling through the potential barrier. Hence, this effect is called the tunneling effect. Single electrons or quasi-particles may tunnel through such barriers, and in so doing cause a potential drop corresponding to the value of the gap voltage. In the band model of electrons, this energy gap corresponds to the separation of two bands representing possible energy states, which separation is measured in units of energy. The energy gap is a property of matter and varies from material to material. However, besides this quasi-particle tunneling effect, there exists a further superconductive tunneling effect involving bound electron pairs. In a superconductive metal, the prevailing charge carriers involved are electron pairs coupled with the lattice by the electron phonon interaction. Such electron pairs are called Cooper pairs. Now it has been predicted by B. D. Josephson that, in sufficiently thin insulating layers between two adjacent superconductive metals, a tunneling effect involving Cooper pairs as carriers occurs. When Josephson tunneling occurs, the insulating material behaves like a superconductive metal but with zero voltage drop. The basic paper is an article entitled "Possible New Effects in Superconductive Tunneling" published by B. D. Josephson in Physics Letters, Vol. 1, No. 7, July 1962, pp. 251-253. The predicted effects have been found and since have been utilized in many technical applications. A good survey of the history, tehcniques, devices and circuits involved is shown in the book by L. Solymar, "Superconductive Tunneling and Applications", published by Chapman and Hall, London, in the year 1972.
Generally josephson elements are called Josephson junctions, i.e., the barrier to be overcome by the tunneling currents consists of a thin oxide layer between two superconductive leads. At least one control line is provided for controlling the switching behavior of the element. A similar switching behavior is also shown by switching elements weakly coupling two superconductors. In such elements, the oxide layer is replaced, e.g., by a weak link. Superconductive Josephson elements are operating at temperatures of a few degrees Kelvin, and they can take two different states depending on the device current. In the region below the so called maximum Josephson current, I.sub.max, a Cooper pair tunneling current flows. This is called pair tunneling, and the voltage drop across the element equals zero. The element is in the superconductive state. When the current value I.sub.max is exceeded, the element switches to another or voltage state. The current flowing in the device under such circumstances is also a tunneling current which, however, involves essentially single charge carriers or quasi-particles. The particle tunneling process is accompanied by a voltage drop which corresponds to the energy gap voltage, V.sub.G.
For simplicity, this state may be called "normal conducting." This so called normal conducting state of a Josephson element should not be confused with the normal conducting state of the cryotron mentioned above. In that case, the normal conducting state is connected with a phase transition of the metal. In the normal conducting state of a Josephson element, the leads remain in the superconducting state, and particle tunneling current flows across the insulating interface. In Josephson elements, a very small voltage drop occurs which results in little heat dissipation. When the device current is reduced after switching into the normal conducting state, a hysteresis effect occurs, i.e., resetting by changing to the superconducting state occurs at a current value considerably below the value I.sub.max. The maximum Josephson current value I.sub.max can be influenced by controlling externally applied magnetic fields which are generated by currents through associated control lines. A graph of the maximum Josephson current value versus the external magnetic field or the control current can take different forms depending on whether or not short Josephson junctions or long Josephson junctions are involved.
Because Josephson elements can take two distinguishable states, they can be utilized in binary circuits. In U.S. Pat. No. 3,281,609, issued oct. 25, 1966, a superconductive switching element utilizing tunneling effects is described. Input current are applied to a Josephson junction. If the current flowing through the Josephson junction exceeds the maximum Josephson current value, the junction switches to the normal conducting state. The resulting output current flows through a load resistance connected in parallel to the Josephson junction. However, it is not shown in this patent how such circuits operate in connection with succeeding logic circuits. U.S. Pat. No. 3,626,391, issued Dec. 7, 1971, describes an example of the application of Josephson elements in binary storage devices. The binary values are represented by the direction of circulating currents in associated superconducting loops. Sensing of the storage binary values is also effected by Josephson elements.
A paper by J. Matisoo entitled "The Tunneling Cryotron -- A superconductive Logic Element Based on Electron Tunneling" describes the application of Josephson elements as logic gate. It appeared in Proceedings of the IEEE, Vol. 55, No. 2, February 1967. This paper describes essentially the switching behavior of a single element. Only one example sketched in a figure suggests controlling the current distribution in both branches of a superconducting loop. Other logic circuits are not shown in that publication.
In U.S. application Ser. No. 267,841 filed June 30, 1973, a binary logic circuit has been proposed using Josephson elements whereby a line terminated with its characteristic impedance is connected in parallel with a Josephson device. In the superconducting state, the current flow essentially through the Josephson junction, but, in the normal conducting state, current is diverted through the line. Succeeding Josephson devices can be controlled by the current in this line. However, this kind of logic circuit is not self-resetting. To reset such Josephson elements to the superconducting state, it is necessary to momentarily interrupt the current flowing through the Josephson device. This resetting mode is time-comsuming and, therefore, decreases the speed of the switching circuits severely. An adjustable delay circuit using that technique is shown in IBM Technical Disclosure Bulletin, Vol. 16, No. 1, June 1973, pp. 347-348. Two such logic circuits with output lines terminated by their characteristic impedance are connected in series, namely an OR gate and an AND gate. The delay period can be adjusted by changing the bias at one input of the AND gate.
A Josephson element as self-resetting logic gate has been proposed in Swiss Pat. application No. 16.755/72 filed Nov. 17, 1972. In this application, switching behavior of a Josephson element is effected by proper design of operating parameters. However, this method requires an extremely high current density within the Josephson junction and tight tolerances which are difficult to meet in the practice.
In the year 1960, a switching circuit operating at room temperature using two identical switching elements connected in series was published. E. Goto et al published the paper "Esaki Diode High-Speed Logical Circuits" in IRE Transactions on Electronic Computers, Vol. EC-9, No. 1, March 1960, pp. 25-29. The paper describes the so called Goto pair or the Esaki diode twin circuit. If this circuit is connected to a constant voltage, it has two stable operating points. that constant voltage feeding the series connection of two tunnel diodes distributes among each circuit element in a different way. Whether the larger voltage drop occurs at the one or the other of the tunnel diodes depends on the control circuit being fed to the node between both diodes at the moment when the circuit is connected to the constant voltage. Pulse operation only is possible because the feeding voltage must be applied anew for every switching operation.
Two Josephson elements connected in series in a level converter circuit were shown in the IBM Technical Disclosure Buletin, Vol. 15, No. 11, pp. 3561-3562, April 1973. That circuit changes unipolar input pulses into bipolar output pulses. The series connection is fed by a pulse current source delivering timing pulses. The unipolar input signals are sent through a control line associated with both Josephson elements. By adjusting portions of the control line, one Josephson element experiences control current in one direction with the other Josephson element experiences control current in the opposite direction. Coincident control and timing pulses cause an output signal of the one polarity, a timing pulse occurring along causes an output signal of the other polarity.